A few days ago, I was asked on facebook to explain what a Blood Moon was. I’d heard the term before, but never as an astronomical term so I went online and googled it. It seems that they are supposed to be incredibly rare events, but that according to the “Blood Moon Prophecy”, there will be a tetrad of them of them in 2014. This is considered to be a hugely significant thing, and is being used to sell a lot of books making various prophecies about world changing events. As usual, the authors variously reference Nostradamus, the biblical Revelation of St John, obscure writings by ancient civilizations and various aspects of both new age and pagan mysticism. Which is all very interesting if you take that sort of thing seriously, but doesn’t answer the question: What is a Blood Moon?Let’s start with the obvious: It must be a Moon that shines in the sky. In recent years, the space journalism community have become keen on publicising folksy names for different apparitions of the Full Moon (Harvest Moon, Wolf Moon, and more), but I don’t think that’s what the Blood Moon crowd are talking about. So what can cause the Moon to appear red?
If you’ve ever caught the full Moon as it rises, very close to the time that the Sun sets, you’ll have noticed that it suffers the same distortions as the Sun: It’s squashed, it appears much larger than usual, and its colours is shifted from white to yellow, orange, or even sometimes red. The size change is a well-studied (but still not understood) optical illusion, but the distorted shape is caused by atmospheric refraction, and the reddening is caused by optical scattering off air molecules and dust particles. When there’s a lot of smoke or dust in the air, the reddening is enhanced. This causes spectacular sunsets, and a deep orange or even ruddy moonrise. This is not particularly common, and since people are seldom looking at the right place at the right time to catch a moonrise, it seems very rare. So that’s one explanation.
But what I think is the most likely answer is simply a total eclipse of the Moon. Because the Earth is quite large, the Moon often passes through its shadow. Thanks to simple geometry, this can only happen when the Moon is full, so people all over Earth get to witness the Full Moon slowly dimming as it enters the penumbra (the blurry fringe part of the shadow), which then transforms into a growing black slice missing from the Moon’s disc. Eventually the Moon vanishes completely, at which point something strange happens. Rather than disappearing entirely, the Moon begins glowing a dull, orange-red colour. The reason? Imagine you’re standing on the Moon, looking up at the sky at the moment of a total lunar eclipse. The Sun has vanished, blotted out by the much larger Earth. The Earth itself is a black disk, but it is ringed with a paper-thin line of orange light: the atmosphere, refracting sunlight. What you’re seeing at this point is the exact same light that you see on Earth during a sunrise or sunset, and this light illuminates the surface of the Moon with a dim orange glow. The exact colour changes from eclipse to eclipse, for the exact same reason that the colours of sunsets vary from day to day.
As it turns out, the second explanation is the correct one. While researching this article, I went and googled “Blood Moon” again, this time taking the time to click the links and read through some of the prophecies (so that you don’t have to). Those that bother, explain that they’re referring to a lunar eclipse which is mysteriously red. The only problem is that these things are written by people who plainly never actually watch the sky for the phenomena that they base their prophecies on: ALL lunar eclipses are red, to some extent or another, and there are several of them every single year!
Oh, and one final thing? What the heck is a “tetrad” of blood moons? It sounds so significant, so scarily prophetic. Well, in the same way that “Trio” and “Trilogy” and “Triad” mean three of something, so “Quartet”, “Tetrad” and “Tetrology” refer to groups of four. It’s not a common word, which I suspect is why the prophets of doom used it!